The invention concerns the field of machines for manufacturing containers made of thermoplastic material by stretching and blow molding a preform that has previously been produced by injection molding.
In particular, such machines are used to manufacture containers, such as bottles made of polyethylene terephtalate (PET).
In such a machine, preforms are used that are appreciably tubular and which are closed at one of their axial ends, the other axial end being open and already has the final shape of the neck of the container.
Each preform is heated in the thermal conditioning oven in order to bring the body of the preform up to a temperature that is higher than the vitreous transition temperature of the thermoplastic material. The preform thus conditioned is transferred to a blow mold in which a cavity is delimited in the shape of the container to be obtained. The preform is placed in the mold so that its open end extends outside the mold. Thus, a blow-molding device can be taken to the level of the neck of the preform in order to inject pressurized air into the preform. Simultaneously, a stretch rod is inserted axially into the preform until it presses against the closed end of the bottom of the preform. The stretch rod thus makes it possible better to control the axial deformation of the preform during the blow molding of the container.
For example, the invention can be implemented in a rotary type stretch blow-molding machine in which the machine includes several blow-molding stations mounted at the periphery of a carrousel that is driven in continuous rotation around its axis. Each blow-molding station includes in particular a blow mold, a blow device, and a stretch device.
According to a known design, the movement of the stretch rod is controlled by a pneumatic cylinder that furnishes the energy required to move the rod, but the stretching speed is regulated by a cam and roller device that enables the axial position of the stretch rod to be perfectly synchronized depending on the angular position of the blow-molding station concerned around the axis of rotation of the carrousel.
These systems are completely satisfactory in terms of operation of the blow-molding process. However, the need has appeared to significantly decrease the overall energy consumption of these blow-molding machines. Indeed, one of the sources of energy consumption of the previously known machines are the stretch rod control cylinders of each blow-molding station that must be fed with pressurized air both to lower and to raise the rod.